Thanks @Tim_Gilbert,
The walls are solid brick, and I’m planning for wood fibre out and in, the rear wall will be external only, so as far as I’m concerned the insulation can be as tick as I want it to be there. The other wall is a ginnel/alley/passage, so can’t have much extra external thickness, so I’m planning 60mm external with a thin silicate render, and then 60mm internal to get to about 0.29U give or take.
Fortunately the ginnel wall is only 1 storey, and the insulation will be going up above the inner ceiling and wrapping over the ginnel, and then a layer and cladding suspended on the ginnel ceiling as well.
So the ginnel wall would be ~0.29, and rear wall likely 0.2 or bellow if practical
(I’ve discussed one idea for the EWI already on Feasibility: EWI with larsen truss + flexible wood fibre batts but may well got for a more standard 150mm solid block system or some other system depending on what my eventual contractor recommends / is comfortable with).